Sticky sessions in HLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the primary purpose of sticky sessions in load balancing?

Sticky sessions are used in load balancing to:

A Ensure a user is always routed to the same server during a session

B Distribute user requests evenly across all servers without preference

C Encrypt user data during transmission between client and server

D Automatically scale the number of servers based on traffic

💡 Hint

Think about user experience consistency during a session.

Sticky sessions ensure that a user's requests go to the same server to maintain session data consistency.

❓ Architecture

intermediate

Which component is essential to implement sticky sessions in a web application architecture?

To implement sticky sessions, which component must be configured to track and route user requests?

A Database replication system

B Content Delivery Network (CDN)

C Load balancer with session affinity enabled

D Firewall with IP filtering

💡 Hint

Consider which part directs traffic to servers.

The load balancer must have session affinity (sticky sessions) enabled to route users consistently.

❓ scaling

advanced

What is a major scalability challenge when using sticky sessions in a distributed system?

Sticky sessions can cause which problem when scaling web servers horizontally?

A Data loss because sessions are stored in a centralized database

B Increased network latency due to session encryption

C Automatic server shutdown due to session conflicts

D Uneven load distribution leading to some servers being overloaded

💡 Hint

Think about how sticky sessions affect load balancing fairness.

Sticky sessions can cause some servers to get more traffic if many users are bound to them, causing uneven load.

❓ tradeoff

advanced

Which tradeoff is most relevant when choosing sticky sessions over stateless session management?

Choosing sticky sessions instead of stateless sessions typically trades off:

A Session consistency for reduced fault tolerance and scalability

B Security for faster response times

C Data encryption for easier debugging

D Lower latency for increased storage costs

💡 Hint

Consider how sticky sessions affect system resilience and growth.

Sticky sessions keep session data on one server, which can reduce fault tolerance and scalability compared to stateless designs.

❓ component

expert

In a high-traffic web system using sticky sessions, which approach best ensures session data availability if a server fails?

To maintain session availability despite server failure in a sticky session setup, you should:

A Store sessions only in server memory without replication

B Use a distributed session store accessible by all servers

C Rely solely on the load balancer to redirect traffic to healthy servers

D Disable sticky sessions and use client-side cookies exclusively

💡 Hint

Think about how to keep session data safe and accessible across servers.

A distributed session store allows any server to access session data, preventing loss if one server fails.

Practice

1. What is the main purpose of sticky sessions in system design?

easy

A. To keep a user connected to the same server during their session

B. To distribute user requests evenly across all servers

C. To store session data in a shared database

D. To encrypt user data during transmission

Solution

1. Step 1: Understand sticky sessions concept

   Sticky sessions ensure that a user's requests go to the same server to maintain session state.

2. Step 2: Compare with other options

   Options B and C describe load balancing and shared storage, not sticky sessions. To encrypt user data during transmission is about security, unrelated here.

3. Final Answer:

   To keep a user connected to the same server during their session -> Option A

4. Quick Check:

   Sticky sessions = same server connection [OK]

Hint: Sticky sessions keep user-server link constant [OK]

Common Mistakes:

• Confusing sticky sessions with load balancing
• Thinking session data is always shared
• Assuming sticky sessions encrypt data

2. Which method is commonly used to implement sticky sessions in a load balancer?

easy

A. Randomly assigning servers per request

B. Encrypting all session data

C. Caching data in a CDN

D. Using IP hashing to route requests

Solution

1. Step 1: Identify sticky session implementation methods

   IP hashing is a common way to ensure requests from the same user go to the same server.

2. Step 2: Eliminate unrelated options

   Encrypting data and caching in CDN do not control session routing. Random assignment breaks stickiness.

3. Final Answer:

   Using IP hashing to route requests -> Option D

4. Quick Check:

   Sticky sessions use IP hashing [OK]

Hint: IP hashing helps keep sessions sticky [OK]

Common Mistakes:

• Confusing encryption with session routing
• Thinking CDN caching manages sessions
• Assuming random server assignment keeps sessions sticky

3. Consider a load balancer using cookie-based sticky sessions. If a user's first request is routed to Server A, where will the second request go?

medium

A. Server B

B. Server A

C. Random server

D. Load balancer itself

Solution

1. Step 1: Understand cookie-based sticky sessions

   The load balancer sets a cookie to identify the server handling the user's session.

2. Step 2: Follow the cookie routing logic

   Subsequent requests with the cookie go to the same server, Server A in this case.

3. Final Answer:

   Server A -> Option B

4. Quick Check:

   Cookie sticky sessions route to same server [OK]

Hint: Cookie keeps user on original server [OK]

Common Mistakes:

• Assuming requests go to different servers
• Thinking load balancer handles requests directly
• Confusing random routing with sticky sessions

4. A system uses IP hashing for sticky sessions but users report inconsistent session behavior. What is the most likely cause?

medium

A. Users have dynamic IP addresses changing frequently

B. Session data is stored in a shared database

C. Load balancer is not configured for SSL

D. Cookies are disabled in user browsers

Solution

1. Step 1: Analyze IP hashing limitations

   IP hashing depends on stable IP addresses to route requests consistently.

2. Step 2: Identify cause of inconsistency

   Dynamic IPs cause requests to appear from different IPs, breaking stickiness.

3. Final Answer:

   Users have dynamic IP addresses changing frequently -> Option A

4. Quick Check:

   Dynamic IPs break IP hashing sticky sessions [OK]

Hint: Dynamic IPs break IP hashing stickiness [OK]

Common Mistakes:

• Blaming shared database for session routing
• Confusing SSL with session stickiness
• Assuming cookies affect IP hashing

5. You design a web app with multiple servers storing session data locally. To ensure users always reach the same server, which approach is best and why?

hard

A. Use IP hashing but allow fallback to any server if IP changes

B. Use a shared database for session storage and random load balancing

C. Use sticky sessions with cookie-based routing to bind users to servers

D. Disable sticky sessions and rely on client-side session storage

Solution

1. Step 1: Understand session storage setup

   Sessions stored locally require users to connect to the same server to access their data.

2. Step 2: Evaluate approaches for consistent routing

   Cookie-based sticky sessions reliably bind users to servers, ensuring session continuity.

3. Step 3: Compare other options

   Shared DB removes need for stickiness but contradicts local storage. IP hashing can fail with IP changes. Client-side storage shifts session responsibility.

4. Final Answer:

   Use sticky sessions with cookie-based routing to bind users to servers -> Option C

5. Quick Check:

   Local sessions need sticky cookie routing [OK]

Hint: Local sessions need cookie sticky routing [OK]

Common Mistakes:

• Choosing shared DB when sessions are local
• Relying on IP hashing despite IP changes
• Ignoring server session storage needs